1. Field of the Invention
This invention generally relates to high frequency circuit interfaces and, more particularly, to a system and method for interfacing a coaxial connector to a substrate with a coplanar waveguide structure.
2. Description of the Related Art
High frequency integrated circuits, such as the circuits needed to support OC-768 data rates in communication systems, must be interfaced with test fixtures during design and/or production test procedures. In order to characterize these integrated circuits (ICs) at very high frequencies, it is important to design test fixtures that minimize the level of signal reflections (mismatch) at all discontinuities. Such discontinuities include the cable-to-test fixture transition and the test fixture-to-IC transition. Good electrical performance is obtained if the transition shows a low level of reflection (return loss), resulting in a minimum insertion loss due to mismatch.
FIG. 1 is a partial cross-sectional view depicting a conventional coaxial-to-outside transmission line transition (prior art). Conventionally, high frequency transitions are achieved by fixing a 1.85 mm or 2.4 mm connector assembly (which consist of a glass bead and a connector) in a metal fixture made of a single piece of metal.
For such an interface it is typical that at least some of the signal energy is reflected where it encounters a conductor medium change. In fact, when a high frequency signal travels through a cable and hits a connector, there will be a small amount of the incoming wave that will be reflected towards the source of the wave. Then, after traveling through the connector, the wave encounters a 50 ohm glass bead, which consists of a metal pin inserted in a glass shell, surrounded by a metal shroud. The wave travels in a coaxial transmission mode into a section of a metal wall that holds the glass bead. On the other side of the wall, an electrical connection is made to a coplanar waveguide (CPW) transmission line. The ground reference of the signal must also be transferred with a minimum discontinuity to ensure a good electrical performance. In the above described configuration, the ground is transferred from the single metal piece to the bottom of the substrate.
The above-described interface works well, but can only be used with a limited class of substrates and fixtures. The substrate must be a single layer dielectric with a groundplane immediately under the dielectric. The fixture is a chassis with a single piece that forms the chassis bottom and the chassis walls. The ground connection between the substrate and the coaxial line is made through the chassis. Thus, the substrate is grounded to the chassis through the bottom layer groundplane. Conventionally, only a single layer board can be interfaced using the conventional chassis/coax interface. As is well known in the art, such thin substrates are typically too thin to polish without breakage. Thus, thin substrates often have burrs along the edges that make the substrate difficult to mount flush against the chassis wall and which promote interface mismatches. Also, an unintentional radius formed between the chassis bottom and chassis wall, caused by imperfect machine milling, also prevents the substrate from flush mounting against the chassis wall, and also promotes interface mismatches.
It would be advantageous if a multi-layered substrate with a top surface coplanar waveguide transmission line could be efficiently interfaced with a coaxial connector.
The present inventions permits a good high frequency electrical connection to be made between a coaxial cable, coming from test equipment for example, and a coplanar waveguide (CPW) transmission line on a substrate top surface, regardless of the thickness of the substrate. This invention is a very high frequency connector launch configuration that receives a signal on one side from a coaxial connector, a 1.85 mm connector for example, and transfers it onto a CPW transmission line. One of the innovations in this transition is in how the ground reference is transferred from the metal wall holding the glass bead, to the CPW transmission line.
The present invention launch design has an assembly feature which permits the enhancement of the electrical performance at the solder connection between the center pin and the attach pad on the CPW transmission line. The metal wall height can be adjusted so that the height of the center pin of the glass bead matches the CPW transmission line despite the substrate thickness.
Accordingly, a substrate interface system is provided for connecting a coplanar waveguide transmission line to a coaxial connector. The system comprises a substrate having a top surface with a coplanar waveguide. The CPW has a transmission line interposed between coplanar groundplanes. A housing wall assembly has an aperture and an interior surface adjacent the substrate coplanar waveguide. A coaxial connector, mounted in the housing wall assembly through the aperture, has a center conductor connected to the coplanar waveguide transmission line, and a ground connected to the housing wall assembly. Extensions are mounted on the wall assembly interior surface, connected to the coplanar waveguide groundplanes.
The substrate need not be grounded to the coaxial connector through a substrate bottom surface groundplane/chassis interface. The substrate may include a plurality of signal trace layers and/or groundplane layers underlying the top surface, and vias proximate to the wall assembly extensions are formed between the coplanar waveguide groundplanes on the top surface and the groundplanes in the layers underlying the surface.
In some aspects of the system, the housing wall assembly is moveable in a vertical plane so that the position of the coaxial connector can be adjusted to connect to the coplanar waveguide transmission line, in response to the substrate thickness. Since the bottom surface of the substrate need not be a groundplane, in some aspects of the system the substrate includes a bottom surface with a plurality of ball grid array (BGA) interfaces.
Additional details of the above-described system and a method for interfacing a coaxial connector to a coplanar waveguide are described below.